The present studies are in continuation of those designed to study the immunologic effect of specific enzyme induced alterations on cell surfaces. We have shown that Vibrio cholerae neuraminidase (VCN) and Concanavalin A (ConA) does not kill cells, but increases the immunogenicity of malignant cells. Because of this property, total regression of firmly established tumors can be induced in syngeneic mice by inoculation of living tumor cells treated with VCN or Concanavalin A. Experiments are designed to study (a) the means by which the immunoregressive potency of treated tumor cells can be increased. The aim is to produce a practical tumor vaccine by which patients can be treated with their own treated tumor cells; (b) to investigate the mechanism by which VCN and Concanavalin A treated tumor cells can increase the immunoregressive tendency in animals bearing syngeneic tumors; (c) to investigate the effects of other agents including Concanavalin A on the immunogenicity and antigenicity of normal and tumor cells.